Diesel engines are typically equipped with a glowplug system. The glowplug system provides a general combustion aid during engine ignition and also during a warm-up phase of the running engine. A key component of this system is the glowplug whose tip can rise up to high temperatures of above 900° C. by means of an electrical to thermal power conversion.
Each cylinder is equipped with one glowplug which is turned on when needed on the base of engine and environmental conditions, typically in cold conditions. Glowplugs function as electrical resistors. Their resistance varies with temperature. As the temperature increases, the internal resistance increases, too.
Different technologies for glowplugs are in use. Glowplugs may be high or low voltage and they may be of different materials, such as metallic or ceramic glowplugs. High voltage glowplugs are typically supplied directly by a vehicle battery. Low voltage glowplugs in contrast, as they have a nominal voltage lower than the battery voltage, typically need a pulse width modulation (PWM) supply to get the correct voltage. Especially the low voltage glowplugs can be controlled easily by connecting the gates of MOSFETS of the PWM to an electronic control unit and controlling the duty cycle of the PWM.
For compression-ignition engines, the most commonly used catalytic converter is the diesel oxidation catalyst. This uses excess O2 (oxygen) in the exhaust gas stream to oxidize CO (carbon monoxide) to CO2 (carbon dioxide) and HC (hydrocarbons) to H2O (water) and CO2. These converters often reach 90% efficacy and help to reduce visible particulates (soot), however they are incapable of reducing NOx as chemical reactions always occur in the simplest possible way, and the existing O2 in the exhaust gas stream would react first. To reduce NOx on a compression ignition engine, the chemical composition of the exhaust must first be changed. Two main techniques are used: selective catalytic reduction (SCR) and NOx traps or NOx Absorbers.
An important development to increase the performance of a catalytic converter is to minimize emissions during the cold start by decreasing the catalyst light-off temperature.
During cold start, the temperature of the catalytic converter is low and the converter is not yet activated. Hence the catalyst light-off temperature at which the conversion of an exhaust gas component reaches 50% is not yet reached, hydrocarbons and CO are thus only not converted to a small extend which is why they contribute significantly to the total emissions in the legislated driving cycles during the first couple of minutes after the engine is started. Special techniques have been developed in order to minimize emissions during a cold start. These fast light-off techniques are either passive systems that employ changes in the exhaust system design, or they are active systems that rely on the controlled supply of additional energy to raise exhaust gas temperature during the cold start.